1. Field of the Invention:
The present invention relates to molding materials which contain the imide group-containing aromatic polyamides and are characterized by substantially improved processibility, and a process for their preparation.
2. Discussion of the Background:
Aromatic polyamides, based on aromatic dicarboxylic acids and aromatic diamines having the general formula EQU H.sub.2 N--Ar--X--Ar'--Y--Ar'--X--Ar--NH.sub.2
are characterized not only by stability at high temperatures and good mechanical properties; but they are also thermoplastically processible (see Elias/Vohwinkel, "New Polymer Materials for Industrial Application", 2nd Edition,. Carl Hanser Verlag 1983, pp. 242 f.). In this formula the X stands for an ether oxygen; and Y, for the sulfonyl or carbonyl group; and Ar and Ar' stand independently of one another for the p- and m-phenylene group. The preparation of these polyamides can be found in the literature.
Broade et al describe for example the preparation of 4,4'-[sulfonylbis(p-phenylenoxy)]dianiline (X=O, Y=SO.sub.2) from p-aminophenol and 4,4'-dichlorodiphenylsulfone and its condensation with aromatic acid chlorides such as terephthalic acid chloride, in organic solvents to form aromatic polyamides having a glass transition temperatures (Tg) ranging from 230.degree. to 320.degree. C. (Polymer Prepr. Am. Chem. Soc. Div. Pol. Chem., Vol 15, 1974, p. 761, and Adv. Chem. Ser., 1975, p. 142; cf. CA, Vol 84, pp. 5530 ff).
DE-OS 35 39 846 discloses aromatic polyamides, which are obtained through polycondensation of aromatic diamines having formula I with aromatic dicarboxylic acids in solvents such as sulfolane in the presence of triphenylphosphite or an acid having the formula H.sub.3 PO.sub.n where n=2, 3 or 4. The preparation processes, which make use of an organic solvent, are expensive from a commercial point of view since the polycondensate has to be precipitated by addition of a so-called "insoluble agent", removed by filtration, liberated from the solvent residues by boiling, dried and concentrated in order to be further processed.
The process of DE-OS 36 09 011 permits on the other hand, the preparation of these polycondensates in the melt of the starting products, thus eliminating the aforementioned drawbacks.
Of course, the melt viscosity of these aromatic polyamides is very high. The result is that during the preparation in the melt and during processing the temperatures must range from 350.degree. to 400.degree. C. Under these conditions, a noticeable decomposition of the polycondensate occurs. Consequently the color and the mechanical properties of the molded products produced deteriorate.
The melt viscosity of highly viscous polymers can drop by adding so-called flow improvers or processing agents (see for example JP-OS 85/245,666, WO 86/03193, JP-OS 85/252,655, JP-OS 85/255, 847, JP-OS 85/255, 848). However, the drawback is that the low molecular weight additives can be leached out and result in stress cracking. With a thermal load these substances are "sweated out" (diffusion to the surface of the molded products) and change the mechanical properties of the polymers.
It is known that at a specific temperature, the melt viscosity is dependent on the molecular weight. Therefore, it should be theoretically possible to obtain polyamides having low melt viscosity by controlling the molecular weight. If, for example, an excess of the dicarboxylic acid or the diamino component is added, products, whose molecular weight is dependent on the molecular ratio of the starting components, are obtained. However, with high processing temperatures it cannot be avoided that the reactive end groups that are present split the chain or undergo other undesired side reactions. It has been observed that the component of which an excess has been added is split off and finally results in a molar ratio of 1:1.
It is also known that special compounds, so-called molecular weight controllers can be added during polycondensation. In Japanese Published Application 86/44,928 aromatic aminocarboxylic acids and monoamides of aromatic diamines are proposed for this purpose. In principle, the same reservations as explained above apply to these compounds. If an aromatic tri- or tetracarboxylic acid is added as a controller, as proposed in the JP-OS 86/44 929, cross-linkages are unavoidable.
It is also known that blends of low and high molecular weight polyamides have a lower melt viscosity. However, if such blends are subjected to the normal temperatures during processing, the result is re-amidations, whereby readily volatile components diffuse out.
Thus in the JP-OS 81/98,260 blends comprising aromatic polyamides having the general formula EQU (--NH--Ar--NH--CO--Ar--CO--)
are claimed, which can be differentiated by their viscosity and consequently also by their molecular weight. However, from the experimental part one can only learn that 2 infusible oligoamides of almost the same molecular weight are mixed.
Thus no molding materials, based on polyamides with the diamine component of formula I and specific molecular weight, exist in prior art, which are thermoplastically processible below 350.degree. C., without resulting in decomposition.